What is Ambient Occlusion?

Instructor: David Gloag
Computer graphics are a hot topic these days, particularly in blockbuster movies. In this lesson, we'll take a look at ambient occlusion, why we would want to use it, and how it works.

Making It Real

Computers are used to fake everything these days. We use them to simulate the weather so we can generate a forecast, to simulate the progress of disease so we can create cures, and to simulate characters that can't possibly be real for the latest movie. Clearly, it's a useful thing to do.

But how do we make them so real? How do we ensure that we truly believe? There is no simple answer to that, and there are many techniques that come into play. One that plays a significant role, from a visual perspective, is ambient occlusion.

What is Ambient Occlusion?

Ambient occlusion is a computer graphics technique that simulates how light falls on an object. It affects all aspects of the object, but is most noticeable in the shadow areas.

The reason for this is because these areas are not affected by direct light, and less significantly by reflected light. Reflected light is light that bounces off of other surfaces in a scene. It comes from the term 'occlusion', which means blocked, and is part of many graphics packages today.

Consider the side-by-side images below:


Ambient Occlusion Example
Ambient-Occlusion


The one on the left was rendered without ambient occlusion, the one on the right, with.

Notice any differences?

The first thing you should notice is that the overall image on the right is darker. That is because ambient occlusion is simulating the effects of light more accurately, and darkening areas like the corners and interiors. The second thing you should notice is that the right image looks more realistic. This is because reality contains both dark and light, so the image appears more realistic to us.

Uses

We use ambient occlusion for two main reasons:

  1. Simulate realism - as mentioned, real-life contains both dark and light. So it makes sense that a 3D rendering would strive for this, particularly those that are trying to be as realistic as possible. As an example, think about the special effects in movies. They are trying to convince you that what you are seeing is real, not created in a computer.

  2. Reduce computation time - calculating shadows for rendering is time consuming, and as such, costs money. So efforts focus on increasing realism, and reducing computational time. As an example, consider the movie special effects mentioned above. Individual frames can take hours to calculate, so every effort is made to reduce that time.

How It Works

Interestingly, ambient occlusion works backwards from every surface in a scene. A line is calculated from every pixel (dot) on the screen that makes up a surface. The line starts out at right angles to the surface, and bounces off every surface it encounters, at an angle equal to the incident angle (like reflection). This continues until the line either reaches the light source, or is blocked.

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